REPORT ON THE ASTEROIDEA. 333 
Chorological Synopsis of the Species. 
Ocean. | Range in Fathoms. Nature of the Sea-bottom. 
Mimaster tizardt : : Atlantic. 516 to 555 Mud. 
Mimaster cognatus  . ; Pacific. 245 to 1325 Blue mud. 
1. Mimaster tizardi, Sladen. 
Mimaster Tizardi, Sladen, 1882, Proc. Roy. Soc. Edin., vol. xi. p. 702; Trans, Roy. Soc. Edin., vol. 
xxx., part ii, p. 580, pl. xxxiv. 
Rays five. R=120 mm.; r=94 mm. R=22 r. The minor radius is thus in the 
proportion of 45 per cent. Breadth of a ray at the base, 58 mm. 
General form large and robust. Marginal contour stellato-pentagonal. Rays short 
and triangular, tapering continuously from the base to the extremity, the breadth at the 
base of a ray greater than the minor radius of the disk, the interbrachial are being 
subacute. 
The abactinal surface is high and inflated over the disk, very gibbous at the base of 
the rays, but flattening towards the extremities. A deep furrow is formed along the 
median interradial line in consequence of the gibbosity, but disappears before reaching the 
centre of the disk. The-actinal surface is more or less convex, but to a comparatively 
slight degree, although the feature is probably largely emphasized by the upward turning 
of the extremities of the rays in consequence of their posture at the time of death. 
Consequent on the curvature of the actinal and abactinal surfaces, the margins are very 
thin and of small dimensions, and are occupied entirely by the double series of small 
marginal plates. The thickness or perpendicular height of the two series of marginal 
plates together is only 4 mm. 
The abactinal surface is covered with a great number of small uniform paxillie, closely 
and equidistantly placed, and with a well-defined space between each, which present no 
definite order of arrangement, excepting in the immediate neighbourhood of the arm- 
angle, where a certain amount of obliquely transverse lineal disposition may be observed. 
The whole of the calcareous portion of the abactinal skeleton is composed entirely of 
paxillee, as in the"Astropectinide. The paxil consist of a cylindrical pedicle, about twice 
as high as broad, expanding slightly at the base, and with the distal extremity rounded 
and clavate, and surmounted by a crown of fifteen to twenty spinelets, which radiate apart 
very slightly and produce a compact form of paxille. The spinelets are short, delicate, 
and slightly taper, about equal in length to the pedicle, and sometimes less, probably 
owing to a certain extent to abrasion. The base of the paxille is quite small and thin 
